The present invention relates generally to a manufacturing method for BiCMOS device, and more particularly to, a manufacturing method for BiCMOS device for static RAMs(Random Access Memories).
Generally, active components which compose logic gates in a semiconductor are divided into two types. One of them is bipolar transistor and the other is CMOS transistor. It is well known that CMOS transistor has a low power consumption and a high integration density, while bipolar transistor has a fast operation speed and high output driving capability.
In recent years, the degree of high performance characteristics of semiconductor integrated circuits has been improved significantly, and this trend is expected to continue in future. And, the degree of high performance is achieved by BiCMOS circuits, where CMOS transistors are used in logic circuits and bipolar transistors are used in peripheral circuits. BiCMOS device is widely used for such products as memory or gate array in Static Random Access Memory (SRAM). This BiCMOS SRAM can minimize the power consumption due to standby current by increasing the load resistance of CMOS cells and can also improve the operating speed by reducing the emitter resistance of bipolar transistor. Thus, BiCMOS SRAM can operate somewhat faster than CMOS SRAM at equivalent power and density for the same geometry, while maintaining the same power consumption and integration density, and thus its application range is extended gradually.
In a prior manufacturing method for BiCMOS SRAMs, a load resistor of a CMOS cell is made of undoped polysilicon layers in order to increase its resistance and an emitter region of bipolar transistor is formed by doped polysilicon layers in order to decrease the resistance. However, since the polysilicon layer is formed at a high temperature, an oxide layer is formed between the emitter region and the polysilicon layer and thus the emitter resistance of bipolar transistor is increased. Moreover, high resistance can not be obtained due to the limited resistance of the undoped polysilicon layers.